After the end of World War I, Hara returned to naval school again for advanced study in torpedo warfare during 1918–19. Then he served as the chief torpedo officer on the destroyer Hakaze, followed by the destroyer Yukaze in 1921, and then the cruiser Ōi in 1922.

Hara attended the Naval Staff College in 1923-24 and then was promoted to lieutenant commander. In December 1926, Hara was assigned to his first command, that of the destroyer Tsuga. He was promoted to the rank of commander in 1929, and then he served as an instructor at several of the naval ordnance schools during the early 1930s. Hara was given command of the gunboat Ataka in 1932, and he was promoted to captain in 1933. During 1933–34, Hara was assigned as the naval attaché at the Japanese Embassy in Washington, D.C.. When he returned to Japan, Hara took command of the cruiser Tatsuta from November 1934 to November 1935, and next he held a number of staff positions in the Imperial Japanese Navy until he was promoted to rear admiral on November 15, 1939.

Following the end of the War in the Pacific, Hara was arrested by authorities of the U.S. Navy and then taken back to Japan to be held in the Sugamo Prison in Tokyo because he had been accused of war crimes. Hara was sent to face a military tribunal on the American island of Guam, and there he was convicted along with other Japanese officers of "neglect of duty in connection with violations of the Laws of War committed by members of their command" for allowing the execution of U.S. Navy aircrewmen who had been captured during air raids on Truk. Since he was the commander of the atoll, Vice Admiral Hara was of course the highest-ranking officer from Truk to be tried for war crimes. Hara was convicted, and he was sentenced to six years in prison. He was then sent back to Sugamo Prison to serve his imprisonment.

Hara's son Nobuaki graduated from the Imperial Japanese Naval Academy just as World War II ended. When Hara was released from prison on April 19, 1951, Nobuaki took him home to a very small house in Tokyo. Hara dedicated the remainder of his life to securing Japanese government pensions and relief for the families of Japanese, Korean, and Taiwanese servicemen imprisoned for war crimes.[3] Hara served as a Councilor of the Ministry of Justice (Japan) until his death at age 74 in 1964.[4]

Shortly after the surrender in August 1945, the commander of allied forces ordered all Japanese swords collected and turned over to the occupation forces. Many of the swords were mass-produced government issue, but some were ancient masterpieces of the swordmakers' art which had been cherished for generations. Many swords were distributed indiscriminately to American servicemen as souvenirs. Hara surrendered his family sword to the American Vice Admiral commanding the Marianas so the sword might be displayed at the United States Naval Academy Museum in Annapolis. Hara was in possession of a second sword which he presented to Rear Admiral Calvin T. Durgin while Durgin was interviewing Hara as part of the United States Strategic Bombing Survey conducted immediately following the war.[3]

In 1959, Hara requested through diplomatic channels return of a family sword belonging to 85-year-old Ryūtarō Takahashi, president of the six-million-member Bereaved Families Association. The sword was one of the great blades forged in Bizen Province in the 15th century. It had been carried by Ryūtarō's son, Hikoya Takahashi. Hikoya had asked Hara to care for the sword while he was assigned to a minesweeping assignment he did not survive. Hara revealed that the sword carefully preserved in the Naval Academy Museum was Takahashi's sword, and the Hara family sword which should have been in the museum was in the possession of Admiral Durgin. Retired Admiral Durgin drove to the museum to correct the error, and the Bizen sword was delivered to the old man who had lost his son.[3]